1. Field of the Invention
The present invention pertains to the field of ophthalmic wavefront sensing and, particularly, to an apparatus and method for aberrometer calibration and accuracy certification.
2. Description of Related Art
A wavefront sensor, often referred to as an aberrometer (which term will be used interchangeably herein), is a device that measures a difference in the optical path of light between a deformed wavefront and an ideal, or reference, wavefront. The measurement, when properly processed, yields values for various aberrations in the optical system that the light propagates through and which deform the wavefront. Although high-energy lasers and astronomical imaging were primary drivers for wavefront sensor development (where the atmosphere itself was the aberration causing optical system), more recent attention has focused on measuring the aberrations of the eye with the goal of improving visual quality. The interested reader is directed to Geary, J M, Introduction to Wavefront Sensors, SPIE Optical Engineering Press (1995); Williams"" U.S. Pat. No. 5,777,719, for more information. These references, to the extent permitted by applicable patent rules and laws, are herein incorporated by reference in their entirety.
The aforementioned Williams"" patent describes a Shack-Hartmann type wavefront sensing instrument that can be used to measure, among other parameters, higher-order ocular aberrations. Many commercial aberrometers incorporate a microlens (lenslet) array and operate on the Shack-Hartmann principle. Other types of aberrometers include the spatially resolved refractometer based on the Scheiner optometer, those based on the Tscherning principle, Skiascopic systems, scanning systems of the Tracey technology type, raytracing devices, and others. All of these aberrometer types are well known in the ophthalmic wavefront sensing art so that a detailed description of these devices is not necessary to understand the invention. Descriptions of these devices can be found, for example, in J. Refractive Surg. 16 (5), September/October 2000.
Ocular wavefront data is increasingly being used to configure ablation algorithms for refractive surgery such as, e.g., PRK, LASIK, and LASEK, and for custom shaping of contact lenses, IOLs, onlays and other vision correcting elements. Successful outcomes to these applications depend upon the validity of the obtained aberration measurement which in turn depends on the correct initial calibration of the aberrometer, and on the correct calibration of the aberrometer when it is used to obtain diagnostic/therapeutic wavefront aberration measurements.
Basically, aberrometers have been calibrated for defocus only using spherical lenses. An Optical Society of America task force set up in 1999 made recommendations for a standard aberrator for calibration of aberrometers. The interested reader is directed to a paper by Thibos, L. N. et al., Standards for Reporting the Optical Aberrations of Eyes (Optical Society of America 1999). Initially, an aberrated model eye was contemplated but was abandoned as too elaborate in view of the different requirements of subjective and objective aberrometers. Instead, a pair of lenses of known spherical power was used as an aberrator, but there were problems with position sensitivity and control. An alternate aberrator design was a trefoil (3rd order aberration) phase plate. Phase plate aberrators, however, are disadvantageous for several reasons, namely: high sensitivity to misalignment due to tilt and decenter, external illumination preferably with a collimated beam, coherence effects such as speckle due to spatial coherence of collimated beam, limit on aberration quantity, high cost in material and time, and others.
Accordingly, the inventors have recognized a need for a method and apparatus that addresses these concerns and others relating to the calibration and accuracy of wavefront measurement and aberrometer operation.
An embodiment of the invention is directed to an improved aberrator or calibration component for an aberrometer. The calibration component preferably comprises a model eye and optionally includes a holder and an alignment tool. The model eye consists of a monolithic, convex anterior surface, refractive cylinder of transparent material for a desired wavelength. The convex, anterior surface can be a sphere, an axisymmetric asphere, or a non-axisymmetric asphere depending upon what aberrations are to be simulated. Alternatively, the anterior surface may be a perfect imaging conic and the posterior surface would have the complex shape that generates the aberrations. Both the anterior and the posterior surfaces can be formed by common fabrication techniques including diamond turning, grinding and polishing, laser machining, etching, molding, and so on. The material of the model eye can include optical glasses, plastics, crystals, and poly-crystalline materials. The model eye according to the invention is advantageous in that arbitrary wavefront aberrations including higher-order aberrations that are both axisymmetric and non-axisymmetric can be simulated accurately, surface alignment is accomplished during manufacture thus eliminating additional alignment, calibration of the model eye can be performed by interferometry and profilometry, the need for external sources or optics is eliminated, and they can be produced inexpensively and in volume through a molding process.
An aspect of the embodiment includes a holder for the model eye. The holder is preferably a self-aligning V-groove mount; however, other types of holders will be apparent to a person skilled in the art. In an associated aspect, an alignment tool is interchangeable with the model eye in the holder.
In another embodiment according to the invention, a method for calibrating an aberrometer for measuring ophthalmic wavefront aberrations includes the steps of providing a model eye having a known wavefront aberration; positioning the model eye along an optical axis of an aberrometer to be calibrated at a location that simulates a wavefront measurement of a patient""s eye; aligning the model eye; and obtaining a wavefront measurement of the model eye. In an aspect of this embodiment, the known wavefront aberration is a defocus error for making a focus calibration of the aberrometer. In another aspect, the model eye has a toroidal convex surface for making a defocus and astigmatism calibration of the aberrometer. In another aspect, the model eye has a non-axisymmetric, convex surface for making a higher-order aberration calibration of the aberrometer. In another aspect, the model eye has a perfect imaging conic-shaped anterior surface and an aberration producing posterior surface.
These and other objects of the present invention will become more readily apparent from the detailed description to follow. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art based upon the description and drawings herein and the appended claims.